The present invention relates to a friction type continuously variable speed changing mechanism and particularly it relates to a friction type continuously variable speed changing mechanism used as a speed increaser in centrifugal blowers, centrifugal compressors, and the like.
As for a device which provides large speed ratios, the 3K planetary gear mechanism is generally known, and FIG. 16 shows a 3K friction type continuously variable speed changing mechanism which has said planetary gear mechanism applied thereto. FIG. 16 shows an application in which this continuously variable speed changing mechanism is used as a speed reducer; however, when it is used as a speed increaser, the input and output shafts 6 and 7 will change places with each other.
In this type of continuously variable speed changing mechanisms, planetary cones 1 are in contact with three traction members, i.e., an input disk 2, a cam disk 3 and a speed changing ring 4, one place for each traction member, totaling three places. The planetary cones 1 are constructed to be dynamically balanced by normal forces acting at said three contact places.
In this case, speed change is effected by moving the speed changing ring 4 in the directions of the input and output shafts. The reason why the balance of normal forces at the three contact places can be maintained irrespective of the axial movement of the speed changing ring 4 is that since the contact portions of the input disk 2 and the planetary cone 1 are formed by surfaces having quadric curvatures, the direction of the normal force acting on the contact portions of the input disk 2 and the planetary cone 1 changes with the axial movement of the speed changing ring 4. Thus, a cone holder 5 has the function of simply holding the planetary cones 1 circumferentially equispaced, not supported by the other members.
In this connection, the construction of the contact portions of the planetary cone 1 and the input disk 2 by using contact surfaces having quadric curvatures increases the influence of the spin. Further, in the case of using the continuously variable speed changing mechanism shown in FIG. 16 as a speed increaser, when the output shaft is rotated at high speed, this leads to the input disk 2 rotating at high speed, resulting in a large moment of inertia of the output shaft system and a large loss of power due to the large peripheral speed and spin in the contact portions, a fact which means that this type of continuously variable speed changing mechanism is not suitable for use with a centrifugal blower which rotates at high speed.
Further, the aforesaid contact portions have to be finished to have a fine surface roughness as by grinding or super-finishing in order to achieve the formation of a satisfactory oil film. Thus, in a case where the contact portions are formed by curved surfaces having quadric curvatures as in the case of the planetary cones 1 of the continuously variable speed changing mechanism shown in FIG. 16, it is difficult to finish them to have a fine surface roughness as by grinding or super-finishing, increasing the manufacturing cost.
Further, though not shown, there is another type of continuously variable speed changing mechanism in which both of the generatrices possessed by the conical surfaces in the contact portions of the input shaft traction portion and the cone and also in the contact portions of the output shaft traction portion and the cone are slightly inclined to the axis of the input and output shafts. It is such shape of the traction portions and an axial force loaded by a pressing spring that produce the normal forces necessary for transmission of power at the contact portions.
The normal force loading mechanism using said pressing spring has the merit of being simple in construction, but the pressing force is constant at all times irrespective of an increase or decrease in transmission power. Since the spring force is set on the basis of the greatest transmission power, it follows that in a case where the transmission power changes in a wide range, normal forces which are excessively large relative to the power will be produced during transmission of the lowest power. As a result, the decrease in transmission efficiency for lower transmission power becomes remarkable.